This invention relates to ultrasonic systems. One particular application of this invention as an ultrasonic scaler is illustrated and described herein; however, the invention, in its broadest form, is not limited to an ultrasonic scaler and may be utilized in other applications and environments.
Ultrasonic scalers and ultrasonic systems in general typically include magnetostrictive or piezoelectric transducer elements for converting an ultrasonic frequency electrical signal, preferably corresponding to the resonant frequency of the transducer, into ultrasonic frequency vibrations which are applied to a work tool. Magnetrostrictive transducers tend to overheat and have a relatively low temperature Curie point and, therefore, are undesirable for scaler and similar applications which require prolonged operation. In many scaler applications, magnetostrictive transducers heat up rapidly, thus causing discomfort to the patient, and quickly reach their Curie point, at which they cease operating. Consequently, scalers equipped with magnetostrictive transducers must be operated infrequently, or in short intervals, and allowed to cool down before they can be used again. Piezoelectric transducers, on the other hand, do not tend to overheat and possess a relatively high temperature Curie point. For these and other reasons, piezoelectric transducers now offer the most economical and effective transducers for scaler and similar applications.
Prior piezoelectric transducers typically include two or more disc-shaped piezoelectric crystals (see U.S. Pat. No. 3,809,977), or a single tubular crystal (see U.S. Pat. Nos. 3,522,801 and 3,645,255). Disc crystals provide a relatively weak, or low powered, transducer. For this reason, at least two disc crystals are necessary in most practical cases in order to obtain acceptable power levels. Multiple disc crystals, of course, increase the complexity of the crystal mounting and vibration transmitting structure and, hence, decrease reliability of the transducer. In the transducer disclosed in the U.S. Pat. No. 3,809,977, for example, at least two disc crystals are energized in parallel by a common electrode and supported by at least three threadably connected components. Additionally, disc crystals are highly sensitive to applied torque, the capacitance thereof changing substantially in response to torque applied to the crystal by the work tool as the latter contacts and is pressed against an object (e.g., a tooth). Disc crystals, therefore, when energized by a circuit particularly sensitive to changes in capacitance (i.e., a high inductive to capacitance ratio, see U.S. Pat. No. 3,596,206), display a rapid decline in vibration frequency from resonant frequency when pressure is applied to the work tool. As the crystal vibration frequency drops off from resonant frequency, of course, the crystal is energized in a much less efficient manner, and the power soon drops off, with concomitant increase in heating. To bring the crystal power back up to an acceptable level, however, it is then necessary to increase the level of electrical power applied to the crystal, thereby producing further undesirable heating.
Although tubular piezoelectric crystals offer to overcome or substantially mitigate these and other disadvantages of disc crystals by providing greater vibration amplitudes, power levels, etc., prior transducers equipped with tubular crystals, such as those disclosed in U.S. Pat. Nos. 3,522,801 and 3,645,255, are prone to failure. Tubular crystals tend to fracture or overheat as destructive tensile stresses accumulate within the crystal when energized. Disc crystals, of course, also tend to fracture or overheat for similar reasons; however, as a practical matter, disc crystals are not operated at power levels and vibration amplitudes comparable to tubular crystals and, hence, they have not experienced the failure problems of tubular crystals in most practical transducer applications. Until this invention, therefore, it has been necessary to sacrifice the superior power levels, vibration amplitudes, etc., offered by tubular crystals for the reduced tendency to fracture or overheat of disc crystals, while tolerating their operating deficiencies.